The invention concerns a device for compensation of field disruptions in magnetic fields of electromagnets with high field homogeneity, in particular, for stabilizing the H0 field of an MR measuring system, comprising at least one field detector for detecting interfering signals, at least one control loop for processing the detected interfering signals, and at least one compensation coil to which the detected and processed output interfering signals are transferred and which generates a correction field for interfering signal compensation.
A device of this type is disclosed e.g. in U.S. Pat. No. 5,302,899 A1, which is used, in particular, in nuclear magnetic resonance (NMR) spectrometers.
Field disruptions produced by additional devices on the spectrometer, by systems and machines associated with building installation (elevators, compressors etc.) or external disturbance sources (streetcars etc.) become an increasing problem in magnet configurations of this type due to the high sensitivity and resolution of the devices.
A field change (ΔB)Z parallel to the static magnetic field B0 of the NMR magnet and in the area of the NMR test substance generates a change Δω in resonance frequency of the magnetic nuclear spins within the test substance, which is proportional thereto, and thereby influences the NMR spectrum. This can be seen from the conventional NMR equation Δω=γ(ΔB)Z. Due to the high spectral resolution and the high sensitivity of modern NMR spectrometers, even minimum disruptive fields of less than 1·10−9 Tesla disturb the NMR spectrum.
It has not been possible up to now to compensate for all relevant field disruptions due to the enormous progress gained in increasing the NMR sensitivity using higher magnetic field strength, the improvement of the field homogeneity within larger measuring ranges, and use of cryogenically cooled measuring probes (cryoprobes) whose sensitivity has increased by factors.
One of these cases which have not been solved up to now concerns the use of refrigerators for cooling superconducting NMR magnets. The unavoidable low-frequency vibrations of such systems produce magnetic field modulations which manifest themselves in the spectrum in the form of sidebands of strong NMR lines. These sidebands are often in the most sensitive area of the NMR spectrum (in the center, in the area of the water line, etc.) and therefore highly disturb the user.
U.S. Pat. No. 4,788,502 describes a superconducting magnet in a cryostat on which a refrigerator is mounted for cooling the cryogenic liquids. The interfering signal from the refrigerator is detected by induction coils or acceleration sensors using a sensor means which is mounted to the refrigerator, and is supplied to the compensation coils via a control device. The compensation coils are preferably mounted to the refrigerator and/or in the room temperature bore (RT bore) of the magnet. The control device contains a coupling matrix with adjustable amplifiers (for “weighting”) and a performance chart. However, experience has shown that measurement at the disturbance source gives only insufficient information about the magnetic field disruptions in the sample volume.
U.S. Pat. No. 5,191,287 A1 illustrates generation of periodic field disruptions in the magnetic spins of the test sample with a test sample that rotates in the inhomogeneous B0 field, thereby producing disturbing sidebands in the NMR signal. These are, however, not compensated for immediately but later on in the NMR receiver where the NMR signal undergoes a second manually tuneable amplitude and phase modulation which generates additional sidebands to compensate for the existing disturbing sidebands. The auxiliary frequencies required for modulation are generated by frequency generators which are synchronized with the rotating test sample. This is a compensation and not a control process, since there is no control loop. A posteriori compensation is not possible or would require great effort, since the person performing spectroscopy can program the course of an NMR measurement largely freely.
Another case concerns disturbances that are produced by use of NMR systems in surroundings which are not optimal, since the number of customers who are prepared to pay for expensive and complex infrastructures (buildings, rooms, etc.) decreases. Floor vibrations are an example therefor, which are produced by systems located in the same building.
U.S. Pat. No. 4,788,502 A1 proposes detecting disruptive fields from remote sources (e.g. trolley cars) via induction coils, and transfers them in an opposite direction to the compensation coils via a control amplifier, to counter-couple the disruptive fields. The induction and compensation coils are primarily located outside of the NMR magnet system and surround it. This method, however, is ruled out when the disturbances are coupled into the magnet system through mechanical vibrations.
U.S. Pat. No. 5,302,899 A1 discloses a method for compensating time-variant field disruptions in NMR, wherein the NMR dispersion signal uX and the NMR absorption signal uY of an NMR reference substance (lock substance) are acquired using a digital NMR field stabilizer (digital lock), from which a correction current is derived which is guided into a field correction coil and compensates for the time-variant field disturbances. A combination of the values uX/uY and 1/uY·(duX/dt) is thereby supplied to a controller with amplifier with single and/or double integration. This substantially produces a PID controller of the measured value uX/uY which provides sufficient compensation of the field disruptions when the controller parameters are adequately adjusted. Satisfactory adjustment of the controller parameters is, however, often not possible in case of disturbances with higher frequency components. When amplification of the controller is small, the generated noise portion is also smaller but at the same time, the control bandwidth also becomes smaller, and the disturbance suppression of the higher frequency components becomes insufficient. Conversely, an increase in amplification increases the control bandwidth, such that an improvement of disturbance suppression could be expected, but at the same time, the noise portion also increases, so that no satisfactory result can be obtained.
It is therefore the underlying purpose of the present invention to propose a device and a method for compensating disruptive fields, which in addition to a high signal/noise ratio (SINO), also ensures improvement of the compensation of periodic field disturbances, in particular, with higher frequency components.